Photoresists are generally photosensitive organic polymers used in semiconductor manufacturing processes to encapsulate portions of a semiconductor which are to be protected from any particular processing step. The following is a brief review of the primary photoresist removal methods.
In virtually all commercially practical prior art processes, the removal of photoresist has required the use of toxic chemicals. Inorganic solutions for removing photoresist typically comprise solutions of sulfuric acid and other components such as nitric acid or peroxide. Organic solvents used for photoresist removal include aceton, trichloroethylene (TCE), methyl ethyl ketene (MEK), and isopropyl alcohol.
Another method that has been tried more recently is plasma stripping in which oxygen is electronically activated and reacted with the photoresist. Unfortunately, plasma stripping has been found to cause RF radiation damage to the semiconductor substrate and also to be prohibitively expensive because it needs to be performed in a vacuum.
It has recently been discovered that irradiation of organic polymers by short pulses of far-UV laser (e.g. 193 nm) light causes ablative photodecomposition of the material. One potential use of this discovery is the etching of photoresists. However, the use of such lasers for photoresist removal is not currently commerically feasible because such laser systems are expensive and product throughput is too slow (because the light emitted by the laser is concentrated on a very small area).
The present invention is an improvement on laser photoablation of organic polymers. In particular, the present invention uses a microwave energy source to induce the generation of high intensity, but noncoherent, far-UV light. The noncoherent UV light has been found to effectively ablate photoresist. The UV radiation is further used to heat oxygen which is blown at atmospheric pressure over the surface of the photoresist being removed. The oxygen reacts with the carries off the photoresist.
By using a microwave energized UV lamp instead of a laser, the cost of irradiating photoresist with UV light is substantially reduced. Ablation is also much faster than in the laser system because a full wafer can be exposed to the UV light from the system's lamp.
The present invention is also substantially better than a system using an electrically energized UV lamp because the microwave energized lamp is more energy efficient and avoids electrode degeneration--which limits the lift of electrically energized lamps.
It is therefore a primary object of the present invention to provide a photoablation system for removing photoresists using a microwave energized UV light source.
A second problem addressed by the present invention is the buildup of very thin layers of hydrocarbons and/or water on semiconductor wafers between processing steps. In the past, people have tried to remove these surface contaminants with acid etches, but etching is often not appropriate. Another technique used in the past has been to deposit a glue-like material, called HMDS before depositing photoresist to overcome the problems caused by surface contaminants.
The present invention removes hydrocarbon and water surface contaminants by providing an apparatus and method for ablating these contaminants from the semiconductor surface. The high intensity short wave ultraviolet light generated by the present invention has been found to ablate and thus remove such surface contaminants, thereby preparing the semiconductor wafer for the next processing step. This process is substantially less expense and less risky than the prior art surface preparation techniques known to the inventor.